<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta http-equiv="X-UA-Compatible" content="IE=edge">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <link rel="stylesheet" href="./small.css" media="screen and (min-width:0px)">
    <link rel="stylesheet" href="./middlescreen.css" media="screen and (min-width:375px)">
    <link rel="stylesheet" href="./bigscreen.css" media="screen and (min-width:750px)">
    <title>引入css</title>
</head>
<body>

    <div class="main">

    </div>
    
</body>
</html>